You mentioned Arthur C Clarke, Max, but none of his books....the 2001 series (2001, then 2010: Odyssey Two, 2061: Odyssey Three and 3001: The Final Odyssey) would have to be included in any list of mine. I appreciate that 2001 is also a film - actually the book and the film were made concurrently - but it stands on its own as a good read.

Horses for courses I guess, or "one man's meat..." but ACC has always been a favourite of mine.
Clarke, Asimov and Heinlein were reckoned to be the "Big 3" and I heard that ACC and Isaac Asimov had a mutual agreement that they were "the two best science fiction writers" which is maybe a bit arrogant, but I'd reluctantly go along with it...

Yeah, if we were on the same continent we'd probably be thumping hell out of each other right about now....
Tell you what....if we ever DO end up in the same end of the same continent, we can sit down and thrash it out over a beer or 3 (like Nietzsche in your philosophy blog)?

I must admit that I did like "Rendezvous with Rama" and I also liked "Cradle" which he wrote with Gentry Lee ... but if you were introducing someone new to science fiction and you knew that they would only read "so many" books ... would you put these on the "must read" list?
Hmmm...

The Rama sequels were interesting and well written. Childhoods End and Songs of Distant Earth also. But Heinlein and others created similar enjoyment of reading, so how can one say one is better than the others?.
Read and enjoy ALL of them. (Old Bull philosophy)

A whole new twist on the debate! Which SF story would one prefer reading while hoisting pints?
Again, ALL of them. Just keep those pints coming...
(joking of course, but I do find it hard to choose a best author or story - they are all great. Ummmm, sort of the same with beer - there is no such thing as a bad beer, some are just better than others)

I am not certain that I would introduce someone to science fiction with the “Crème de la Crème”. Some good SF might scare off a novice.
To "Dune" and "The Moon Is a Harsh Mistress", I might add "The Mote in God's Eye" (Larry Niven and Jerry Pournelle) and "The Dispossessed" (Ursula K. LeGuin).
One of the strengths of science fiction (and a reason that some prefer the term speculative fiction) is the ability to ask "what if" in a way that can often be less limited by local cultural biases.
I admit that I also liked Gordon R. Dickson; the feel-good stories were fun, the sad stories were touching (sometimes painful), and some had fairly deep ponder-worthy content. I do not think I would class any of them in the best of the best category, though. Likewise, Lois McMaster Bujold's works are very enjoyable, but I doubt I would list any among the ten best SF works (though I would be more willing to recommend such to a novice because they are fun).

"The Mote in God's Eye" was really good, as was "Footfall" and "Lucifer's Hammer"...
...actually, now I come to really think about it, I think I would include all three of these books in my "must read" list...

How could I forget Heinlein's "The Puppet Masters" (for an intermediate / advanced audience) and "Between Planets" and "Star Beast"(for a younger audience)?
The surprising thing to me is that with so many science fiction authors and books out there, there are so few on my "Must Read" list...
It's like when you are planning a party. You may have a handful of friends in the "inner circle", but things grow exponentially as you widen the selection... The same thing applies here, if we were to open this topic up to include "jolly good reads" then we could list hundreds of books... the trick is to restrain ourselves to the best of the best...

If I had to pick only a very few books to introduce someone to science fiction, I'd choose the following (while noting that all the other books mentioned are all good reads)
For the younger audience: Heinlein's "Rocket Ship Galileo" and "Have Spacesuit Will Travel" with the rest of the Heinlein books for younger audiences held in readiness for when the bug hits :)
For intermediate readers I think Asmov's Foundation trilogy would be a good starting point.
For older readers "The Moon Is a Harsh Mistress" and perhaps "Enders Game" or maybe "The Mote in God's Eye"
In addition to the books already mentioned, Micheal Crichton's "Andromeda Strain" or "Terminal Man" might make good inclusions on the list for older readers.

"Have Spacesuit Will Travel" was unique in that it was intended for younger readers and emphasized the importance of education, human values, self-learning, and getting along with each other. It also pointed out many human shortcomings.

I've read them both -- I'm currently reading "Micro" -- they are all "OK" -- but not "Must Read" (I'm a hard man to please :-)
I must admit that out of these three, "Timeline" did make me think the most...

I would expect anyone who started out with Heinlein would be hard to please :)
One of the things that has happened with this series is that I now have a new list of books to look for when I visit the local used book store...

I'll have to re-read "Childhood's End" because I can't remember the "feel" of it in enough detail to make a decision.
I've never heard of "Wreck of the River of Stars" but the reviews are good on Amazon and I've just added it to my "Wish List"

My "favorite" of RAH, ACC, IA, and AN depended on which I was reading at the time, and I frequently had several by different authors going at once. They were all my favorite for different reasons.
How could you leave out Jules Verne? He invented the field.
Someone else not mentioned so far, Alfred Bester "The Demolished Man".

I left out Jules Verne and H.G.Wells on purpose because I think their writings would be a bit awkward / tedious for a modern audience.
Having said this I Should have included John Wyndham (and I will in Part 2 :-)

Wow, nice listings! RAH was my first SF author when I was in 7th grade (yeh Coles JH library). I had read every book out of RAH, IA, and ACC by the end of 9th grade in'72 . But I have to mention RAH's "Friday" as a must read. I also consider "the Hunger Games" by Suzanne Collins (and the remaining 2 books in the trilogy) as great writing for current SF. I could not stop reading it as with most any RAH book.

I would go with "Contact" by Carl Sagan, "His Master's Voice" by Stanislaw Lem and "Blindsight" by Peter Watts. Three totally different visions of the first contact. "Blindsight" is by the way IMO the best SF book I've read and if anyone wants to take a shot at more recent SF I recommend it wholeheartedly.

"Contact" is a great book...
I've read a lot of Stanislaw Lem, but not "His Master's Voice", and I've never heard of "Blindsight", but the reviews on Amazon are very good -- I've added both of these to my Wish List...

'Blindsight' is hard as nails SF and some people find it unattractive because of that. Heck, it even has references given at the end - as it is the case with scientific papers. Not as optimistic as 'Contact' though. If you decide to read it, I'm very curious of your opinion, as you seem to be a fan of the rock-solid SF classics.

I will read it, but it may be a while because I already have a really big pile of books waiting for my attention plus a humongous "Wish List" on Amazon...
... life is good ... there are folks out there in the world that don't like reading and don't know what to do without a TV running in the background...